Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications. The important parameters of an absorptive attenuator are: attenuation as a function of frequency; return loss; and stability over time and temperature.
It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. In many cases, it is impossible or impractical to remove the temperature variations in some Radio Frequency (RF) components. For example, the gain of many RF amplifiers is temperature dependent. In order to build a system with constant gain, a temperature-dependent attenuator is placed in series with the amplifier. The attenuator is designed such that a temperature change that causes the gain of the amplifier to increase will simultaneously cause the attenuation of the attenuator to increase such that the overall gain of the amplifier-attenuator system remains relatively constant.
Another consideration is to assure that the impedance of an attenuator remains substantially constant over its operating range of interest. The above-referenced U.S. Pat. No. 5,332,981 describes a first such attenuator and U.S. Pat. No. 7,119,632 describes a second such attenuator.
FIG. 1 which is reproduced from FIG. 1 of the '632 patent is a schematic representation of a series absorptive attenuator 100 that includes three resistors R1, R2, R3 separated by transmission line sections 112 and 113. The resistor R1 is provided in series between a transmission line 111 and the transmission line section 112. The resistor R2 is provided in series between the transmission line section 112 and the transmission line section 113. The resistor R3 is provided in series between the transmission line section 113 and a transmission line 114. In one embodiment, the resistors R1-R3 are thick-film resistors; in another embodiment resistors R1-R3 are thin-film resistors. The transmission line section 112 is one-quarter wavelength long at a first desired center frequency. The transmission line section 113 is one-quarter wavelength long at a second desired center frequency. In one embodiment, the first desired center frequency will be the same as the second desired center frequency. In one embodiment, the resistors R1-R3 are temperature-dependent resistors (thermistors), where the resistance of each thermistor varies with temperature according to a temperature coefficient. In one embodiment, the resistors R1 and R3 have the same resistance and temperature coefficient. In one embodiment, the resistance of the resistor R2 is twice the resistance of the resistors R1 and R3. In one embodiment, the temperature coefficient of the resistor R2 is twice the temperature coefficient of the resistors R1 and R3. In one embodiment, the transmission lines 111-114 have the same characteristic impedance. FIG. 1 shows three resistors for purposes of illustration. One of ordinary skill in the art will recognize that two, three, four or more resistors separated by transmission line sections can be used.
The attenuator 100 behaves as a lossy transmission line, as the resistors R1-R3 absorb a portion of the energy propagating between the transmission line 111 and the transmission line 114. If the resistance of the resistors R1-R3 is different from the characteristic impedance of the transmission lines 111 and 114, then the resistors R1-R3 will produce undesired reflections on the transmission lines 111 or 114.
By making the transmission line sections 112 and 113 one quarter wavelength long at a desired frequency, the reflections from the resistors will cancel at the desired frequency, and thus the reflections on the transmission lines 111 and 114 will be reduced or eliminated at the center frequency and in a band about the desired center frequency.
One of ordinary skill in the art will recognize that two, three, four or more resistors separated by transmission line sections can be used. The transmission line sections can be of different length and/or different characteristic impedance (e.g., different width). In one embodiment, standard microwave filter design techniques are used to design the attenuator by selecting the parameters that do not vary with frequency (e.g., the number of resistors, the lengths and impedances of the transmission lines, etc.), and then determining the resistor values at a number of temperatures to match the desired attenuation-temperature profile over the desired bandwidth. Once the resistances at a number of temperatures are known, the temperature coefficients of each resistor are selected to produce the desired temperature profile in each resistor.
In one embodiment, the resistors R1-R3 are thick film resistors are produced by inks combining a metal powder, such as, for example, bismuth ruthenate, with glass frit and a solvent vehicle. This solution is deposited and then fired onto a ceramic substrate which is typically alumina but could also be beryllia ceramic, aluminum nitride, diamond, etc. When the resistor is fired, the glass frit melts and the metal particles in the powder adhere to the substrate, and to each other. This type of a resistor system can provide various ranges of material resistivities and temperature characteristics can be blended together to produce many different combinations.
The resistive characteristics of a thick film ink is specified in ohms-per-square. A particular resistor value can be achieved by either changing the geometry of the resistor or by blending inks with different resistivity. The resistance can be fine-tuned by varying the fired thickness of the resistor. This can be accomplished by changing the deposition thickness and/or the firing profile. Similar techniques can be used to change the temperature characteristics of the ink.
The temperature coefficient of a resistive ink defines how the resistive properties of the ink change with temperature. A convenient definition for the temperature coefficient of the resistive ink is the Temperature Coefficient of Resistance (TCR) often expressed in parts per million per degree Centigrade (PPM/C). The TCR can be used to calculate directly the amount of shift that can be expected from a resistor over a given temperature range. Once the desired TCR for a particular application is determined, it can be achieved by blending appropriate amounts of different inks. As with blending for sheet resistance, a TCR can be formed by blending two inks with TCR's above and below the desired TCR. One additional feature of TCR blending is that positive and negative TCR inks can be combined to produce large changes in the resulting material.